A molecular analysis of archaeal community structure and activity in grassland rhizosphere soil
The prokaryotic domain Archaea represents one of the three major evolutionary lineages of cellular life. Cultivated Archaea are represented by organisms limited to environments of extreme temperature, salinity or anoxia. However, molecular surveys have recently revealed uncultivated Archaea to be globally distributed and active in a variety of marine, freshwater and terrestrial habitats, with a lineage associated with the hyperthermophilic Crenarchaeota kingdom being the most ecologically diverse. The diversity of rhizosphere soil Archaea from three grassland types, associated with different management practices, was examined at a site in the Border region of Scotland, using 16S rRNA and rDNA methodologies. DGGE and sequence analysis revealed the archaeal community to be dominated by two distinct lineages of nonthermophilic Crenarchaeota, with sequences associated with methanogenic Euryarchaeota being retrieved only after anaerobic enrichment. Analysis of the distribution of archaeal communities in the rhizosphere demonstrated a large amount of spatial variability within and between replicate plots. However, grassland management was demonstrated to affect both community structure and activity and, the same dominating band or ribotype was present in all three grassland soils examined. Analysis of 16S rRNA-derived amplicons from managed and natural grasslands at sites in the north of England and the north of Wales also revealed crenarchaeotes to be the predominantly active Archaea. Again, management specific differences were observed, with one dominating ribotype the same as that at the Scottish site. Soil microcosm experiments examined the effects of three environmental parameters associated with the transition of natural to managed grassland; an increase in pH, increased urine addition due to intensive sheep grazing, and fertiliser application. In direct contrast to the bacterial community, profiles associated with the active members of the archaeal community were highly stable exhibiting little change to perturbation.